Bacterial fruit blotch (also known as watermelon fruit blotch) is a serious disease that strikes cotyledons, leaves and young fruit of cucurbits particularly, watermelon and cantaloupe. Bacterial fruit blotch is caused by the bacterium, Acidovorax avenae subsp. citrulli. Fruit is susceptible to infection during flowering and fruit set and infection does not spread via contact during fruit transit or storage.
Symptoms of the bacterial fruit blotch appear on the fruit shortly before harvest, which is often long after the period of infection. Infected areas (lesions) on the rind of watermelon fruit appear water-soaked or oily. Cracks sometimes develop in the more advanced lesions that may contain brown, gummy ooze. Such cracks allow other organisms to enter and cause fruit decay. In melons, the external symptoms on fruit can be very limited, however, the causal agent is invasive and can cause serious degradation of the fruit flesh.
Symptoms can also occur on the foliage. The earliest symptoms are water-soaking between the veins on the underside of the seedling cotyledons (seed leaves) (Thomas Isakeit, “Bacterial Fruit Blotch of Watermelon,” L-5222, Texas Agricultural Extension Service, The Texas A& M University System, 6/99). These areas eventually dry up and die. Id. On true leaves, the disease can form distinctive brown, elongated lesions on and next to the veins, which can also appear water-soaked. Id. The bacteria also produce brown, circular spots on the leaves although, similar lesions can be caused by a number of other agents. Id. Nonetheless, the large lesions on fruit and angular lesions on the true leaves are distinctive for bacterial fruit blotch. Id.
The bacterial fruit blotch pathogen typically spreads to areas where it has not occurred before via seed (seed borne). Environmental conditions play a key role in symptom development and disease severity. Id. Rain helps spread bacteria onto developing fruit. Overhead irrigation also contributes to disease development. High temperatures (higher than 90° F.) and high humidity are also a factor.
Infection of commercial melon fields with bacterial fruit blotch during the 1990's has, in extreme cases, caused losses of about 90% of growers' fields. Such losses have been devastating to the industry. Therefore, there is a need in the industry for sensitive methods of identifying bacterial fruit blotch in seed lots prior to planting to insure against the presence, contamination and spread of the disease in the field.